Note: In the following write up I'll reffer to the DM/GM as the Referee or Ref' in a vain attempt to avoid causing offence to fanatical believers in eaither term
Alternity is now sadly discontinued :-(. Fortunately the d20 system is pretty similar and Wizards claim to be working on a series of SF games for the d20 system. I'm sure many Ref's out there will be working on their own d20 SF games and due to the similarity between systems conversion isn't much of a problem.
Professions -> Classes
This really depends on your Referee and the campaign you are entering. A free wheeling Ref' might just recomend writing down your Alternity Profession and using it as a Class, this might also be the case if the setting is remaining the same and only the rules are changing. This is something to work out with your Referee.
Alignment
You'll have to remember to pick an Alignment. Look at your PCs history and ask youself; how has your character acted? ("How did my PC act after the sugeon botched that roll? hmmm...N...E...")
Ability Equivilents
The Whooo this is a breeze!
Alternity -> d20
Strength=Strength
Dexterity=Dexterity
Constitution=Constitution
Inteligence=Inteligence
Will=Wisdom
Personality=Charisma
Ability Stat Conversions
This is a little more complicated. There are a number of possible meathods:
The +0 Option
If the campaign you are entering is a 'realistic' or 'gritty' one your Referee may ask you to simply carry over your stat. This is harsh, but may make sense in certain cases. I would recomend using this method for any Alternity characters venturing into Ravenloft, it hightens the fear and tension and strongly discourages any attempts to turn the game into a Hack and Slash fest.
So Alternity Strength 13 = d20 Strength 13.
The +2 Option
As a general rule adding two points to an Alternity stat in order brings it more or less in line with d20. This works if you are in a hurry, such as quickly converting a non-central NPC from a module. Players will probably prefer a more exact system for their hard earned stats.
So Alternity Strength 13 = d20 Strength 15.
The Table Option
The +2 option does make it somewhat dificult for a human to start off with d20 Strength 18. I think that Alternity humans are actally weaker than their d20 cousins because that is the feel of the game, more gritty and realistic. But if you want the full range, the table below lists what I consider to be general workable equivilent stats, this is subjective, I have no secret formula:
Alternity -> d20
4=4
5=6
6=7
7-8=10
9=11
10=12
11=13
12=14
13=16
14=18
15=19
16=20
+1=+1
So Alternity Strength 13 = d20 Strength 16
Ability Modifiers: However you convert your stats, once you have you must then look up your new stat on the Ability Modifier chart.
Skills
This is a nasty area. Alternity uses skills whenever possible, d20 doesn't. D20 has those confusing little things called Feats. I would recomend the following course of action:
1: Go through your list of Alternity Skills and note down the basic weapons & armour you are skilled in. You automatically gain the feats needed to have the ability to use that equipment.
2: Compare the d20 and Alternity Skill lists. Find those skills which have obvious correlations. You gain those d20 Skills. A Broad Skill will grant you 1 d20 Skill level. Add one d20 Skill level per additional Specialist skill rating.
3: List all Skills that do not correlate (there are no equivilents at all). You'll have to ask your Referee if these Skills are alowable in his/her campaign. If you get the go ahead you can gain them as above.
Perks, Flaws and Feats
Perks generaly translate quite nicely into Feats. Peruse the Feat listing, use any obvious correlations, if not there shouldn't be any problems with simply writing your Perk in as a new Feat generaly a +1 Step can be used as a d20 +1 without causing too many discrepancies. Flaws will have to be included as interesting and unique effects of the character's background.