Summarize:

For regular verbs ending in -er, you form the past participle by removing the -er ending and instead using an e. For example, the past participle of parler (to speak) is parle. For regular verbs ending in -ir, you don't have to remove the entire ending. Just remove the r so that the verb ends with an i. For example, the past participle of the verb choisir (to choose) is choisi. If you have a regular French verb ending in -re, you simply remove the -re ending and replace it with a u to form the past participle. For example, the past participle of vendre (to sell) is vendu. Just as in English, French has a number of irregular verbs that may have endings similar to regular verbs, but are conjugated differently. Their past participles tend to be irregular as well, so you just have to memorize them as you come across them. In some cases you can find patterns. For example, most irregular verbs that end in -oir have a past participle that ends in u. The past participle of "voir" (to see) is "vu;" the past participle of "vouloir" (to want or to wish) is "voulu." Most irregular verbs that end in -re have a past participle that ends in -is. For example, the past participle of "comprendre" (to understand) is "compris;" the past participle of "apprendre" (to learn) is "appris."
Replace -er with e. Remove the r from -ir verbs. Exchange a -re ending with a u. Memorize irregular verbs.